The present invention relates to signal sampling systems and more particularly to signal processing methods and architecture for capturing and digitizing transient signals where a high frequency rate of sampling is needed, but the events leading to the signal samples occur at a significantly lower frequency than the sampling frequency.
In a variety of fields it is useful to sample signals at a high rate in order to determine the time variations in intensity, phase or other features that occur in a waveform over time. One example of this sampling is the use of fluorometers, where precise fluorescence intensity data may be gathered as part of a variety of detection tasks. In particular, it is desirable to record the time rates of decay of the fluorescence produced by one or more fluorophores as a result of illumination by a short pulse of light. Another example is in observing transmission phenomena, where changes between a transient input signal applied to a system and the corresponding transient output signal are of interest. A further example is radar and similar systems in which a signal is emitted and it is of interest to capture the signal resulting from a reflection. If the sampled signals are digitized, it greatly facilitates analysis of the resulting data.
There exist systems that perform continuous digitizing of sampled analog data. If the sampling rate must be high, then because all the components involved in sampling and analog to digital (A to D) conversion must operate at the same high rate to avoid massive analog memory requirements, the systems are very expensive. In some circumstances the high sampling rate needs to occur only during a short sampling window corresponding to a transient event. This provides an opportunity to sample at a high rate and perform digitizing at a slower rate. This limits memory and may permit cheaper components to be used for slower functions. Such systems are known as Fast-in Slow-out (FISO) systems, e.g., U.S. Pat. Nos. 4,833,445 and 6,091,019.
With most FISO systems challenges remain. When sampling data from an event occurs at near gigahertz (or higher) rates, large amounts of samples must be stored and processed. Often the interval for processing the samples is limited by the occurrence of the next event and its corresponding sampling activity. Thus, there are trade-offs between and among the sampling rate, the duration of the sampling window, the speed of the A-to-D conversion, the frequency of occurrence of the sampling window, the amount of data that can be collected and the amount of digitized data that can be passed on for downstream processing. If the duration of the sampling window is short, then the window on the event observed is narrow. Thus, when a high sampling rate is required, a shortening of the sampling window can help limit the downstream data processing but may also mean that less than adequate observations are made. Designs achieving increases in speed or amount of data collected almost always involve cost increases or power demands that limit applications for the design.
It is unusual to capture long waveforms using a high sampling rate. Those who do it use relatively expensive components. In the fluorescence situation, the most expensive elements are the light source (usually a laser) and the digitizer, usually a digital storage oscilloscope. The relatively high cost has limited the use of such equipment.
It would be desirable to develop a system and method for capturing analog samples of data signals that could provide a high rate of sampling and efficient delivery of digital data derived from the captured samples. Other desirable features are a high degree of accuracy and lower cost than conventional devices.
The subject invention, in one embodiment, is an apparatus for capturing and digitizing at least one analog signal derived from an event with a signal duration that is short compared to the interval between consecutive analog signals comprising:
two or more memories each capable of storing of a sequence of analog samples of one of two or more analog signals derived from the event;
a trigger for triggering the sampling and storage in the two or more memories of a sequence of analog samples to occur at a 0.5 gigahertz or higher rate;
means communicating with the memories for selectively initiating the read out of the analog samples in memories;
an analog to digital converter for receiving each analog sample read out from each of memories and producing in parallel from the analog samples corresponding digitized sample values;
a digital signal processor for operably controlling parameters of the analog to digital converter and receiving the digitized sample values; and
an output stage under control of the digital signal processor for outputting the digitized sample values, such that the receiving and conversion and the output of digitized sample values is completed during the interval between consecutive analog signals.
Another embodiment of the subject invention is a method as performed by the preceding apparatus.
While multiple embodiments are disclosed, still other embodiments of the subject invention will become apparent to those skilled in the art from the following detailed description. As will be apparent, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the subject invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.